Delta printer twisting beginning of print

giostark

As "bot"(user ) , but with less skill , I try to give some help. I have lost so much time back this damn trap of a delta.
I'm agree with him about :

I find that the calibration is good at hinting at these major tower geometry problems, but it's best to sort them out manually rather than let the calibration "fix it."

@kjiwa said in Delta printer twisting beginning of print:

@dc42 I do not believe I am using steel-cored belts. When you say the that one of my towers is not moving linearly, are you suggesting that there is some curvature to its movement?

In terms of its assembly, I measured the distance between each pair of towers at the top, middle, and bottom, and the horizontal distances were all identical to within 0.5 mm over height of the printer. So this leads me to believe that the towers are fairly parallel to each other. The angular offset should be no more than 0.05 degrees, if even that much.

I once again measured each of the diagonal rods and they were all close to 215 mm, but I measured them while the machine was assembled, so my confidence in them being exactly the same is reduced. That said, the rods (and towers) were cut and provided to me by Johann R, who designed the Mini Kossel, so I expect they were measured and cut precisely.

I want to note that I didn't have these problems with my RAMPS board, and nothing else about the printer has changed since the upgrade to the Duet 2 WiFi. Here's a sample print from when I still had the RAMPS board on: https://imgur.com/a/L1Hyo4S. I had also printed a 3DBenchy and it came out really nicely.

-The tower could have some torsion or be not perpendicular to the bed. If you put a drawing T-square against the bed and the tower , the two sides adhere perfectly? (I before blocked the two T-squares with bi-adhesive against an other square)

-I don't know how much difference are allowed in the towers geometries but for the rods must stay in the 0.1mm :
https://duet3d.dozuki.com/Wiki/Calibrating_a_delta_printer
-Did you moved the printer? My delta is so heavy that if the bed is not flat to the ground the tower can flex a little.
-About the belts : the guy tested the GT2 and iron core saw that the last one elongation is 1/3 of the fiberglass core.
https://www.reddit.com/r/3Dprinting/comments/61j9et/i_tested_some_gt2_belts_to_failure_on_a_tensile/dffh799/
http://mark.rehorst.com/misc/corexy/dual belts Z 4kg.jpg
-I would give the full power at the motors in the config.g (1.5A).

If you launch 10 strait calibration , how much big is the deviation between each others? If is too much must be a mechanical trouble. (just because the autocal found different results each time it was performed and it have to recalculate everything) If they are equal +/- 0.01x mm it look like the compensation is working. (ex could be better but is fair enough)

With a digital caliper the cube measure 5cmx5cm or is a rhomboid instead? By the pictures seems irregular.
If there is a mechanical issue the autocal can come even worse that leaving a low factor calibration in the bed.g.
You could try also S8 instead S4 but just avoid the factors that calculate the rods length. BUT you must be sure about the length of the rods!
(the only thing that the autocal make it wrong is calculate the rods length)
I designed a lower parallelepiped (30x3x2 cm) for check the real goodness of the autocal. In small and centered objects the calibration could seems good enough but if you print large object you will find really soon if some issue is still there.

For my calibration.
-endstop all 3 at the same length on the carriage (measured with digital caliper).

-I put to zero the M666 in the override.g , save and reboot.
-I calculate the Z height and stored it in the M665 (override.g) , save and reboot and check if it is good (G1 Z0)
-I use the bed.g with 10 out point , 6 internal. The radius as much large you can till the rods are parallel to the towers reach the best results. X3 (tree pass at least). If the build is accurate the second an third pass should be more or less the same.
http://www.escher3d.com/pages/wizards/wizardbed.php
-Now if the geometries are good enough the autocal should work.
-You can refine with the mesh grid.

The process is a chain, all the rings must be right.

Ah.... this print that look good enough (0.1mm height) is printed with a NOT well calibrated printer. But because in the middle of the bed, all the issue are less noticeable.
ps The ultra squished 1st layer depend on the glue that was irregular (I used a sponge)
Keep us aware of your new discovers

kjiwa

Thanks for the tips @giostark. But as discussed earlier, I'm fairly certain the rod length is accurate, and as I had mentioned, I was able to make good quality prints before when I was using the RAMPS board.

We're at a point where the twist is gone from my prints. It is a small print, but this indicates that my towers are likely straight, and not bowed or highly angled.

The issue I am seeing now is with layer shifting in the calibration cube where the letters begin and end. What could be causing that? There are entire sections of the print that are straight. It's only when there is a change in the object that the layers shift a little bit.

Calibration cube side profile

bot

The angle that the firmware was adjusting for was the tower's placement around the circle. IE the Y tower was not evenly spaced compared to the other two.

giostark

What happen if you try to print this object on one side of the bed? Maybe close the border?
The shift is there but the right side is more bended of the left one. The center is more narrow of few mm (from the picture).
Please try also to give the full power at the motors.

giostark

OT/
Bot , so this is you !!! (just kidding )
https://www.reddit.com/r/3Dprinting/comments/fohhwo/muahahaha/?utm_source=share&utm_medium=web2x
/OT

kjiwa

I put some more work into my build and managed to get rid of the twist. My prints are coming out very nicely now. Thank you everyone for the help.

I believe my issues came from two major areas:

I mentioned earlier that my bed had some tilt to it. I was aware of the tilt, but I was under the impression that the Duet controller could compensate for it. The documentation states that a level bed "isn't essential, because RepRapFirmware can compensate for [it]." Anyway, I redesigned my bed mounting parts and gave the bed a lot more support.
I am using FSRs, but I had not done a great job of calibrating them. This meant that in some cases the nozzle had to depress the bed a lot to generate a reading. When I redesigned the bed mounts, I also used the opportunity to calibrate the FSRs and to make sure that their ambient resistance was in a range where they would be sensitive enough to detect approximately 500g of mass.

I still have some other settings to dial in now, but at least I can move on from worrying about the straightness of my prints. Thanks again!

kjiwa

I was a bit premature in declaring victory. I continue to have issues with layer shifting and autocalibration. At this point I've tried almost everything except replacing the steppers. I've taken everything apart, measured parts to ensure they are sized accurately, and replaced the timing belts, pulleys, and idlers. Still, no improvement to the layer shifting.

What's really confusing me is why the autocalibration gives me such wild results between runs. Here is the output from the last few invocations of G32:

Diagonals 215.000:215.000:215.000, delta radius 105.620, homed height 236.757, bed radius 70.0, X 1.482°, Y 0.880°, Z 0.000°
Diagonals 215.000:215.000:215.000, delta radius 110.722, homed height 237.256, bed radius 70.0, X -14.987°, Y -7.550°, Z 0.000°
Diagonals 215.000:215.000:215.000, delta radius 106.415, homed height 236.746, bed radius 70.0, X 1.289°, Y 0.492°, Z 0.000°
Diagonals 215.000:215.000:215.000, delta radius 105.221, homed height 236.595, bed radius 70.0, X -1.524°, Y -0.171°, Z 0.000°
Diagonals 215.000:215.000:215.000, delta radius 106.281, homed height 236.804, bed radius 70.0, X 1.278°, Y 0.430°, Z 0.000°
Diagonals 215.000:215.000:215.000, delta radius 106.814, homed height 236.817, bed radius 70.0, X 0.138°, Y 1.348°, Z 0.000°
Diagonals 215.000:215.000:215.000, delta radius 106.660, homed height 236.546, bed radius 70.0, X 0.512°, Y -0.868°, Z 0.000°

This is just a sample from today. There are times when the control board detects nearly 0 degree offsets, but most of the time it's something like 0.5 degrees. The second one you see is a bit crazy though -- it thinks my build has a nearly -15 degree angle! This is my first ever build, so I may not have done a great job, but I can assure you that nothing about my build is this inaccurate. I have also watched every moving part very closely, but my eye is unable to detect any sort of wobbling or movement.

@dc42 You mentioned two things that I thought might be the culprit: my belts and the diagonal arms. I tested the arms and they are all the same size (measured as best as I could with my tools). I also wondered if I had abused my belts, so I replaced them in case there was any stretch or wear.

Any other suggestions? Could it be my linear rails? Or might the motors have some issues?

kjiwa

Just to close out this thread... since this is not a Duet-related issue I've moved the thread to the deltabot forums on Google Groups: https://groups.google.com/forum/#!msg/deltabot/MKpY_hvHnLE/vij7Q4LOAwAJ.

Thanks again for all of the suggestions here.

dc42

@kjiwa said in Delta printer twisting beginning of print:

Any other suggestions? Could it be my linear rails? Or might the motors have some issues?

Loose pulley grub screw?

kjiwa

@dc42 Thank you. I also replaced and tightened down the pulleys. They seem firm and in the right positions.

kjiwa

I think the next thing I'm going to try is a new set of arms. It seems like the preassembled rods on AliExpress are about the same as the cost of materials, so I'm going to give them a shot as well as making some new ones of my own. I'll update the thread once they arrive (which will not likely be any time soon given lead times from China these days).

giostark

Can you do something like this?
https://forum.duet3d.com/topic/6951/not-yet-solved-tevo-little-monster-duetwifi-errors/137
In my experience, unlucky (but I have to admit that the adversity helped me to understand the things) the cheap china product are similar to the roulette.
In my specific case, and I hope truly is not your too:

holes of the towers not aligned with the holes of the top and base.(so basically everything was a little twisted)
rods of different length (supersonic mess)
bltouch not accurate as stated.
the bed glass is glued not parallel to the aluminium frame , so it look like always bend. (I had to use spring under the bed and with the digital caliper pinch the glass and the frame and measure the right height between the towers)
I had to dismount everything 4 time before to understand all the tolerances.
I'm still working on it.
The Haydn rods are really sturdy and accurate but not cheap. And...not suited for hight temp working (hot chamber with 50/60+ degree)

kjiwa

Thanks @giostark.

Frame alignment: When I disassembled the printer and began reassembling it, I checked for alignment on the top and bottom frames. I also tightened them down with belt clamps. I think they are in good shape.

Rod length: I'm beginning to suspect my rods more and more. Currently the measuring tools I have can only give me accuracy to within +/- 0.2 mm, so I don't think I can detect errors to the degree stated in the post you linked.

I did not get them from China, but when I inspected them more closely I found one that had a loose ball joint and another one that had the ball joint glued in at a slight angle. When I repaired them, I measured them all to be the same to within that 0.2 mm margin, but I'm guessing that perhaps that is too wide of a margin for accuracy.

BLTouch: I'm not using a BLTouch.

Bed: This was previously an issue, but I've since fixed the bed mounting and the glass is very secure and level on the frame now.

giostark

Ok,
Also I suggest you to verify those alignments.
Those must be at the same equal distance.
Doing this I reached that the X and Y of M665 are gone from several decimal to some cents.
In those trap we must gain as much precision everywhere
In my case for obtain the more possible equal distance , I was not able to keep the tower aligned with the top frame.